In semiconductor processing, as in layer growth technology, and in coating of optical substrate processes, it is important to know the quality of intimate contact between film and substrate. Among the well known methods of quality testing of the interface, optical microscopy is often used.
In the method of optical microscopy, a judgement on the adhesion quality is made relying on optical effects on the interface, such as reflection, transmission and scattering of visible light. This method provides evaluation of transparent objects on the scale of several microns. Another method of visual testing is obtained by electron microscopy, which performs similar testing with electrons having wavelengths 3 orders of magnitude less than visible light and thus providing 3 orders of magnitude increased spatial resolution. Since both of the above methods respond primarily to electron susceptibilities of materials, neither allows obtaining direct information on intimate contact of film and substrate lattice structures.
Photothermal excitation and probe techniques became one of the first attempts to get direct information regarding the surface and interface, (U.S. Pat. No. 4,513,384, to Rosencwaig.) Rosencwaig teaches the process of periodic excitation of the sample interface by focused laser beam, generating of a thermal wave inside the sample, probing of the sample by the second focused laser beam, and measuring the reflectivity variation of the probe beam induced by the thermal wave. This method responds directly to the thermal properties of material (thermal conductivity) with spatial resolution limited by the size of the focused laser beam (several microns.) The further development and extensive usage of this method were described in the papers titled "Photoacoustic and Photothermal Phenomena II" Editors: J. C. Murphy et al., Springler Series in Optical Sciences, v.62,1990.
However, low spatial resolution is a principal limitation of the thermal wave method. For detection of amplitude and phase variation of the thermal wave one has to consider both exciting and probe laser beams focused to spots with sizes less than the distance between these spots (ideally, to geometrical point sources). Theoretical limitation for minimal spot size is the laser wavelength--approximately equal to 0.5 micron--which yields several microns as a theoretical spatial resolution limit for thermal wave method.
The aforementioned prior art method does not satisfy the need for much higher resolution for many processes such as coatings or crystal growth. The spatial resolution required for these processes has to be of the order of magnitude of an elementary cell or several elementary cells, which value might be tens or hundreds of Angstroms.
As soon as the variations of intimate contact quality are considered on a scale compared to elementary cell size (or, on the so called mesoscopic scale), the location of perfect or deteriorated areas on the interface is not important. It is crucial for evaluation of the quality of interfaces to measure the fraction of perfect contact, that is the ratio of perfect contact area to the entire area of a contact (interface) and to estimate an average size of mesoscopic contact area.
For example, in the process of crystal growth on a substrate the mismatch between film and substrate lattices which is accumulated along the interface eventually produces dislocations or sublattices (see, for example, Jan H. van der Merve, W. A. Jesser, Material Science and Engineering, A113, 85,1988 and V. L. Pokrovskii and A. L. Talapov, Sov. Phys. JETP,51,134,1980). As a result, the mismatch causes reduction of intimate contact in the range from several to several hundred unit cells, which counts for several hundred Angstroms.
Another possible problem arises with respect to adhesion of a coating to a substrate. Encompassed in this category is a wide variety of processes, such as: sputter deposition, emulsion coating, MOCVD and the like. The macroscopic deteriorations of intimate contact between the deposited coating (that usually strongly absorbs visible light) and the substrate, on the scale of several microns, may be evaluated by the thermal wave method. However the most important question to be solved and which is achieved with the present invention is the determination of the fraction of mesoscopically deteriorated area on the interface.
Analysis of wetting processes, e.g. spreading of a liquid substance over the surface of a solid substrate (P. G. de Gennes, Rev. Mod. Phys. 57,827 (1985)) creates the problem of evaluation of intimate contact between the spreading liquid layer and the substrate. This kind of evaluation is especially important for biophysical and biochemical systems. Intimate contact between a spreading liquid layer and a solid substrate may be adversely effected by impurities, local substrate imperfections etc. Definition of the precise fraction of unwetted substrate is an important characteristics that can not be obtained by the prior art.
Accordingly, it is therefore an object of the subject invention to provide a new method for evaluating quality of intimate contact between film and substrate on the mesoscopic scale. The method of the invention improves spatial resolution by two orders of magnitude compared to the existing methods.
It is another object of the subject invention to provide a new method for evaluating, with spatial resolution on the mesoscopic scale, the quality of a wide variety of coatings deposited on a wide variety of substrates.
It is a further object of the subject invention to provide a new method for evaluating the fraction of unwetted surface of liquid on a substrate with spatial resolution of several hundred Angstroms.